The present invention relates generally to control methods for induction motors and, more particularly, to induction motor speed control systems for use in aircraft.
Induction motors have been employed for various purposes in aircraft products. It has, for example, been proposed to use an induction motor system in an Advanced Ducted Propulsion (ADP) fan pitch control system having a dedicated generator mounted to the engine in order to provide electric power to drive the induction motor.
Previously, high performance induction motor controls for similar applications have employed a field orientation technique called "vector control". The objective of vector control is to control the induction motor as a shunt wound d.c. motor by controlling the field excitation (the magnetizing current) and the torque producing current separately. In other words, the stator current of the induction motor is resolved into separately controllable torque producing and flux producing components. This can be accomplished, for example, by locking the phase of the reference system such that rotor flux is aligned entirely in the d-axis. The main issue in vector control methods is the acquisition of the fundamental flux wave of the motor rotor representing the synchronous frame of reference for field orientation. Based upon the methods of obtaining rotor flux information, the vector control technique is typically termed either "direct" or "indirect" sensing.
Direct sensing methods, for example, can employ Hall effect sensors or other magnetic induction elements to measure the air gap flux vector. However, such d.c. current sensors can be relatively expensive and unreliable in the high temperature environments of aircraft engines.
Indirect sensing methods are typically based on a voltage or a current (slip frequency) model of the induction motor. The current model based indirect method estimates the rotor flux vector from stator currents and rotor speed or position. In other words, slip frequency is estimated as a function of rotor time constant and torque and the flux producing components of the stator current. A disadvantage of this method is that a rotor speed/position sensor is required, such as a resolver mounted on the induction motor shaft. Thus, the motor control costs are greater while system reliability may decrease since extra control hardware and connections are involved.
On the other hand, the voltage model based indirect method obtains the rotor flux vector by integrating the induced voltage detected directly via sense coils or calculated indirectly from stator currents and voltages. Sense coils are often less sensitive to variation in the motor operating parameters. However, the accuracy of this method is limited by the accuracy of integration, which is typically worse at zero and low speeds. At low speed operations an open loop slip control system can be used, but when the induction motor is already in motion (such as when it is being driven by the prime motor) the initial speed is not known. Thus, the appropriate slip command for an open loop control is not known.
Another disadvantage of prior vector control systems has been in the employment of PWM inverters to produce variable voltage and variable frequency power to the induction motor stator windings. In typical aircraft engine environments, such PWM inverters must include EMI filters to decrease noise to acceptable levels.
Accordingly, it is an object of the present invention to overcome these and other disadvantages in the prior art. The present invention achieves that by detecting rotor speed through the use of sense coils on the stator teeth. Voltage ripples caused by the rotor slots is detected by combining the signals of several such sense coils so that the fundamental voltage signal is cancelled or rejected and only the ripple voltages remain, a signal proportional to rotor speed. Several alternative methods for implementing that technique are discussed herein. At low induction motor speeds, the present invention filters out the voltage ripples and incident third harmonics and instead estimates rotor speed from the fundamental voltage. When the induction motor is already being driven by a prime mover, the present invention selects a proper initial frequency command for initial motor start by identifying rotor speed from the PWM inverter. Further, the present invention provides for using a generator to control voltage to the induction motor, with a PWM inverter to control frequency signals to the induction motor.
Other objects, advantages and novel features of the present invention will now become readily apparent to those skilled in the art from the following drawings and detailed descriptions.